Windows Internals Tour - Unict€¦ · Windows Internals Tour Windows Processes, ... – Multiple...
Transcript of Windows Internals Tour - Unict€¦ · Windows Internals Tour Windows Processes, ... – Multiple...
Windows Internals Tour
Windows Processes, Threads and
Memory Andrea Dell’Amico – Microsoft Student Partner
Roadmap
Processes and Threads
– Processes, Threads, Jobs
and Fibers
– Processes and Threads
Data Structures
– Create Processes
– Scheduling
Memory
– Memory Manager
Features and
Components
– Virtual Address Space
Allocation
– Shared Memory and
Memory-Mapped Files
– Physical Memory Limits
Windows Processes
• What is a process? – Represents an instance of a running program
• you create a process to run a program
• starting an application creates a process
– Process defined by: • Address space
• Resources (e.g. open handles)
• Security profile (token)
• Every process starts with one thread – First thread executes the program‟s “main” function
• Can create other threads in the same process
• Can create additional processes
Windows Threads
• What is a thread? – An execution context within a process
– Unit of scheduling (threads run, processes don‟t run)
– All threads in a process share the same per-process address space • Services provided so that threads can synchronize access to shared
resources (critical sections, mutexes, events, semaphores)
– All threads in the system are scheduled as peers to all others, without regard to their “parent” process
Processes & Threads
• Why divide an application into multiple threads? –Perceived user responsiveness,
parallel/background execution
–Take advantage of multiple processors • On an MP system with n CPUs, n threads can
literally run at the same time
–Does add complexity • Synchronization
• Scalability
Jobs
• Jobs are collections of processes – Can be used to specify limits on CPU, memory, and security
– Enables control over some unique process & thread settings not available through any process or thread system call
• E.g. length of thread time slice
• Quotas and restrictions: – Quotas: total CPU time, # active processes, per-process CPU
time, memory usage
– Run-time restrictions: priority of all the processes in job; processors threads in job can run on
– Security restrictions: limits what processes can do
Job
Processes
Process Lifetime
• Created as an empty shell
• Address space created with only ntdll and the
main image unless created by POSIX fork()
• Handle table created empty or populated via
duplication from parent
• Process is partially destroyed on last thread
exit
• Process totally destroyed on last dereference
Thread Lifetime
• Created within a process with a CONTEXT record
– Starts running in the kernel but has a trap frame to return to user mode
• Threads run until they:
– The thread returns to the OS
– ExitThread is called by the thread
– TerminateThread is called on the thread
– ExitProcess is called on the process
Why Do Processes Exit? (or Terminate?)
• Normal: Application decides to exit (ExitProcess)
– Usually due to a request from the UI
– or: C RTL does ExitProcess when primary thread function (main, WinMain, etc.) returns to caller
• this forces TerminateThread on the process‟s remaining threads
• or, any thread in the process can do an explicit ExitProcess
• Orderly exit requested from the desktop (ExitProcess)
– e.g. “End Task” from Task Manager “Tasks” tab
– Task Manager sends a WM_CLOSE message to the window‟s message loop…
– …which should do an ExitProcess (or equivalent) on itself
• Forced termination (TerminateProcess)
– if no response to “End Task” in five seconds, Task Manager presents End Program dialog (which does a TerminateProcess)
– or: “End Process” from Task Manager Processes tab
• Unhandled exception
9
Fibers
• Implemented completely in user mode – no “internals” ramifications
– Fibers are still scheduled as threads
– Fiber APIs allow different execution contexts within a thread
• stack
• fiber-local storage
• some registers (essentially those saved and restored for a procedure call)
• cooperatively “scheduled” within the thread
– Analogous to threading libraries under many Unix systems
– Analogous to co-routines in assembly language
– Allow easy porting of apps that “did their own threads” under other systems
Windows Process and Thread
Internals
Data Structures for each
process/thread:
• Executive process block
(EPROCESS)
• Executive thread block (ETHREAD)
• Win32 process block
• Process environment block
• Thread environment block
Process environment
block
Thread environment
block
Process block (EPROCESS)
Thread block (ETHREAD)
Win32 process block
Handle table
...
Process address space
System address space
Process
• Container for an address space and threads
• Associated User-mode Process Environment Block (PEB)
• Primary Access Token
• Quota, Debug port, Handle Table etc
• Unique process ID
• Queued to the Job, global process list and Session list
• MM structures like the WorkingSet, VAD tree, AWE etc
Thread
• Fundamental schedulable entity in the system
• Represented by ETHREAD that includes a KTHREAD
• Queued to the process (both E and K thread)
• IRP list
• Impersonation Access Token
• Unique thread ID
• Associated User-mode Thread Environment Block (TEB)
• User-mode stack
• Kernel-mode stack
• Processor Control Block (in KTHREAD) for CPU state when not running
Quota Block
Exit Status
Primary Access Token
Process ID
Parent Process ID
Exception Port
Debugger Port
Handle Table
Process Environment Block
Create and Exit Time
Next Process Block
Image File Name
Process Priority Class
Memory Management Information
EPROCESS
Kernel Process Block (or PCB)
Image Base Address
Win32 Process Block
Process Block Layout
Dispatcher Header
Processor Affinity
Kernel Time
User Time
Inwwap/Outswap List Entry
Process Spin Lock
Resident Kernel Stack Count
Process Base Priority
Default Thread Quantum
Process State
Thread Seed
Disable Boost Flag
Process Page Directory
KTHREAD . . .
ETHREAD
Create and Exit Time
Process ID
Thread Start Address
Impersonation Information
LPC Message Information
EPROCESS
Access Token
KTHREAD
Timer Information Pending I/O Requests
Total User Time
Total Kernel Time
Thread Scheduling Information
Synchronization Information
List of Pending APCs
Timer Block and Wait Blocks
List of Objects Being Waiting On
System Service Table
TEB
KTHREAD
Thread Local Storage
Kernel Stack Information
Dispatcher Header
Trap Frame
Thread Block
Process Environment Block
• Mapped in user space
• Image loader, heap manager, Windows system DLLs use this info
• View with !peb or dt nt!_peb
Image base address Module list
Thread-local storage data Code page data
Critical section time-out Number of heaps
Heap size info
GDI shared handle table OS version no info Image version info
Image process affinity mask
Process heap
Thread Environment Block
• User mode
data structure
• Context for
image loader
and various
Windows DLLs
Exception list Stack base Stack limit
Thread ID Active RPC handle
LastError value
Count of owned crit. sect. Current locale
User32 client info GDI32 info
OpenGL info TLS array
Subsyst. TIB
Fiber info
PEB
Winsock data
Process Creation
• No parent/child relation in Win32
• CreateProcess() – new process with primary thread
BOOL CreateProcess( LPCSTR lpApplicationName, LPSTR lpCommandLine, LPSECURITY_ATTRIBUTES lpProcessAttributes, LPSECURITY_ATTRIBUTES lpThreadAttributes, BOOL bInheritHandles, DWORD dwCreationFlags, LPVOID lpEnvironment, LPCSTR lpCurrentDirectory, LPSTARTUPINFO lpStartupInfo, LPPROCESS_INFORMATION lpProcessInformation)
UNIX & Win32 comparison
• Windows API has no equivalent to fork()
• CreateProcess() similar to fork()/exec()
• UNIX $PATH vs. lpCommandLine argument
– Win32 searches in dir of curr. Proc. Image; in curr. Dir.;
in Windows system dir. (GetSystemDirectory); in Windows dir.
(GetWindowsDirectory); in dir. Given in PATH
• Windows API has no parent/child relations for processes
• No UNIX process groups in Windows API
– Limited form: group = processes to receive a console event
Opening the image to be executed
What kind of application is it?
Run CMD.EXE Run NTVDM.EXE Use .EXE directly
Run NTVDM.EXE Run POSIX.EXE Run OS2.EXE
Win16 (not supported on
64-bit Windows)
Windows
OS/2 1.x MS-DOS .EXE, .COM, or .PIF
MS-DOS .BAT or .CMD
POSIX
Win32 (on 64-bit Windows)
Use .EXE directly (via special Wow64 support)
If executable has no Windows
format...
• CreateProcess uses Windows „support image“
• No way to create non-Windows processes directly
– OS2.EXE runs only on Intel systems
– Multiple MS-DOS apps may share virtual dos machine
– .BAT of .CMD files are interpreted by CMD.EXE
– Win16 apps may share virtual dos machine (VDM)
Flags: CREATE_SEPARATE_WOW_VDM
CREATE_SHARED_WOW_VDM
Default: HKLM\System...\Control\WOW\DefaultSeparateVDM
– Sharing of VDM only if apps run on same desktop under same security
• Debugger may be specified under (run instead of app !!)
\Software\Microsoft\WindowsNT\CurrentVersion\ImageFileExecutionOptions
Flow of CreateProcess()
1. Open the image file (.EXE) to be executed inside the process
2. Create Windows NT executive process object
3. Create initial thread (stack, context, Win NT executive thread object)
4. Notify Windows subsystem of new process so that it can set up for new proc.& thread
5. Start execution of initial thread (unless CREATE_SUSPENDED was specified)
6. In context of new process/thread: complete initialization of address space (load DLLs) and begin execution of the program
The main Stages Windows follows
to create a process
Open EXE and create selection
object
Create NT process object
Create NT thread object
Notify Windows subsystem
Set up for new process and
thread
Start execution of the initial
thread
Return to caller
Final process/image
initialization
Start execution at entry point to
image
Creating process
Windows subsystem
New process
CreateProcess: some notes
• CreationFlags: independent bits for priority class -> NT assigns lowest-priority class set
• Default priority class is normal unless creator has priority class idle
• If real-time priority class is specified and creator has insufficient privileges: priority class high is used
• Caller„s current desktop is used if no desktop is specified
Creation of a Thread
1. The thread count in the process object is incremented.
2. An executive thread block (ETHREAD) is created and
initialized.
3. A thread ID is generated for the new thread.
4. The TEB is set up in the user-mode address space of the
process.
5. The user-mode thread start address is stored in the
ETHREAD.
Scheduling Criteria
• CPU utilization – keep the CPU as busy as possible
• Throughput – # of processes/threads that complete their execution per time unit
• Turnaround time – amount of time to execute a particular process/thread
• Waiting time – amount of time a process/thread has been waiting in the ready queue
• Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (i.e.; the hourglass)
How does the Windows scheduler
relate to the issues discussed:
• Priority-driven, preemptive scheduling system
• Highest-priority runnable thread always runs
• Thread runs for time amount of quantum
• No single scheduler – event-based scheduling code spread across the kernel
• Dispatcher routines triggered by the following events: – Thread becomes ready for execution
– Thread leaves running state (quantum expires, wait state)
– Thread„s priority changes (system call/NT activity)
– Processor affinity of a running thread changes
Windows Scheduling Principles
• 32 priority levels
• Threads within same priority are scheduled following the Round-Robin policy
• Non-Realtime Priorities are adjusted dynamically – Priority elevation as response to certain I/O and
dispatch events
– Quantum stretching to optimize responsiveness
• Realtime priorities (i.e.; > 15) are assigned statically to threads
Windows vs. NT Kernel Priorities
– Table shows base priorities (“current” or “dynamic” thread priority may be higher if base is < 15)
– Many utilities (such as Process Viewer) show the “dynamic priority” of threads rather than the base (Performance Monitor can show both)
– Drivers can set to any value with KeSetPriorityThread
Win32 Process Classes
Realtime
High
Above
Normal
Normal
Below
Normal
Idle
Win32 Time-critical 31 15 15 15 15 15
Thread Highest 26 15 12 10 8 6
Priorities Above-normal 25 14 11 9 7 5
Normal 24 13 10 8 6 4
Below-normal 23 12 9 7 5 3
Lowest 22 11 8 6 4 2
Idle 16 1 1 1 1 1
Kernel: Thread Priority Levels
16 “real-time” levels
15 variable levels
Used by zero page thread
Used by idle thread(s)
31
16
0
i
15
1
Special Thread Priorities
• Idle threads -- one per CPU
– When no threads want to run, Idle thread “runs”
• Not a real priority level - appears to have priority zero, but actually runs “below” priority 0
• Provides CPU idle time accounting (unused clock ticks are charged to the idle thread)
– Loop:
• Calls HAL to allow for power management
• Processes DPC list
• Dispatches to a thread if selected
• Zero page thread -- one per NT system
– Zeroes pages of memory in anticipation of “demand zero” page faults
– Runs at priority zero (lower than any reachable from Windows)
– Part of the “System” process (not a complete process)
Single Processor Thread Scheduling
• Priority driven, preemptive – 32 queues (FIFO lists) of “ready” threads
– UP: highest priority thread always runs
– MP: One of the highest priority runnable thread will be running somewhere
– No attempt to share processor(s) “fairly” among processes, only among threads
• Time-sliced, round-robin within a priority level
• Event-driven; no guaranteed execution period before preemption – When a thread becomes Ready, it either runs immediately or is
inserted at the tail of the Ready queue for its current (dynamic) priority
Thread Scheduling
• No central scheduler! – i.e. there is no always-instantiated routine called “the scheduler”
– The “code that does scheduling” is not a thread
– Scheduling routines are simply called whenever events occur that change the Ready state of a thread
– Things that cause scheduling events include: • interval timer interrupts (for quantum end)
• interval timer interrupts (for timed wait completion)
• other hardware interrupts (for I/O wait completion)
• one thread changes the state of a waitable object upon which other thread(s) are waiting
• a thread waits on one or more dispatcher objects
• a thread priority is changed
• Based on doubly-linked lists (queues) of Ready threads – Nothing that takes “order-n time” for n threads
Scheduling Data Structures
Dispatcher Database
Process
thread thread
Process thread thread
Default base prio Default proc affinity Default quantum
31
0
Ready summary Idle summary 31 (or 63) 0 31 (or 63) 0
Base priority Current priority Processor affinity Quantum
Bitmask for non-empty ready queues Bitmask for idle CPUs
Scheduling Scenarios
• Preemption
– A thread becomes Ready at a higher priority than the running thread
– Lower-priority Running thread is preempted
– Preempted thread goes back to head of its Ready queue
• action: pick lowest priority thread to preempt
• Voluntary switch
– Waiting on a dispatcher object
– Termination
– Explicit lowering of priority
• action: scan for next Ready thread (starting at your priority & down)
• Running thread experiences quantum end
– Priority is decremented unless already at thread base priority
– Thread goes to tail of ready queue for its new priority
– May continue running if no equal or higher-priority threads are Ready
• action: pick next thread at same priority level
Scheduling Scenarios
Preemption
• Preemption is strictly event-driven – does not wait for the next clock tick
– no guaranteed execution period before preemption
– threads in kernel mode may be preempted (unless they raise IRQL to >= 2)
• A preempted thread goes back to the head of its ready queue
18 17 16 15 14 13
Running Ready
from Wait state
Scheduling Scenarios
Ready after Wait Resolution
• If newly-ready thread is not of higher priority than the running
thread…
• …it is put at the tail of the ready queue for its current priority
– If priority >=14 quantum is reset (t.b.d.)
– If priority <14 and you‟re about to be boosted and didn‟t already have a
boost, quantum is set to process quantum - 1
18 17 16 15 14 13
Running Ready
from Wait state
Scheduling Scenarios
Voluntary Switch
• When the running thread gives up the CPU…
• …Schedule the thread at the head of the next non-empty “ready” queue
to Waiting state
18 17 16 15 14 13
Running Ready
Scheduling Scenarios
Quantum End (“time-slicing”)
• When the running thread exhausts its CPU quantum, it goes to the end of its ready queue – Applies to both real-time and dynamic priority threads, user and kernel
mode • Quantums can be disabled for a thread by a kernel function
– Default quantum on Professional is 2 clock ticks, 12 on Server • standard clock tick is 10 msec; might be 15 msec on some MP Pentium systems
– if no other ready threads at that priority, same thread continues running (just gets new quantum)
– if running at boosted priority, priority decays by one at quantum end (described later)
Running Ready 18 17 16 15 14 13
Basic Thread Scheduling States
Ready (1) Running (2)
Waiting (5)
voluntary switch
preemption, quantum end
Priority Adjustments
• Dynamic priority adjustments (boost and decay) are applied to threads in “dynamic” classes
– Threads with base priorities 1-15 (technically, 1 through 14)
– Disable if desired with SetThreadPriorityBoost or SetProcessPriorityBoost
• Five types:
– I/O completion
– Wait completion on events or semaphores
– When threads in the foreground process complete a wait
– When GUI threads wake up for windows input
– For CPU starvation avoidance
• No automatic adjustments in “real-time” class (16 or above)
– “Real time” here really means “system won‟t change the relative priorities of your real-time threads”
– Hence, scheduling is predictable with respect to other “real-time” threads (but not for absolute latency)
Priority Boosting
To favor I/O intense threads:
• After an I/O: specified by device driver – IoCompleteRequest( Irp, PriorityBoost )
Other cases:
• After a wait on executive event or semaphore
• After any wait on a dispatcher object by a thread in the foreground process
• GUI threads that wake up to process windowing input (e.g. windows messages) get a boost of 2
Common boost values (see NTDDK.H) 1: disk, CD-ROM, parallel, Video 2: serial, network, named pipe, mailslot 6: keyboard or mouse 8: sound
Thread Priority Boost and Decay
Priority
Base Priority
Run Wait Run
Preempt (before quantum end)
Run
Priority decay at quantum end
Boost upon wait complete
Round-robin at base priority
quantum
Time
Windows Memory Management
Fundamentals
• Classical virtual memory management
– Flat virtual address space per process
– Private process address space
– Global system address space
– Per session address space
• Object based
– Section object and object-based security (ACLs...)
• Demand paged virtual memory
– Pages are read in on demand & written out when necessary (to make room for other memory needs)
Windows Memory Management
Fundamentals
• Lazy evaluation
– Sharing – usage of prototype PTEs (page table entries)
– Extensive usage of copy_on_write
– ...whenever possible • Shared memory with copy on write
• Mapped files (fundamental primitive)
– Provides basic support for file system cache manager
Memory Manager Components
• Six system threads
– Working set manager (priority 16) – drives overall memory management policies, such as working set trimming, aging, and modified page writing
– Process/stack swapper (priority 23) – performs both process and kernel thread stack inswapping and outswapping
– Modified page writer (priority 17) – writes dirty pages on the modified list back to the appropriate paging files
– Mapped page writer (priority 17) – writes dirty pages from mapped files to disk
– Dereference segment thread (priority 18) – is responsible for cache and page file growth and shrinkage
– Zero page thread (priority 0) – zeros out pages on the free list
MM: Working Sets
• Working Set:
– The set of pages in memory at any time for a given process, or
– All the pages the process can reference without incurring a page fault
– Per process, private address space
– WS limit: maximum amount of pages a process can own
– Implemented as array of working set list entries (WSLE)
• Soft vs. Hard Page Faults:
– Soft page faults resolved from memory (standby/modified page lists)
– Hard page faults require disk access
• Working Set Dynamics:
– Page replacement when WS limit is reached
– NT 4.0: page replacement based on modified FIFO
– From Windows 2000: Least Recently Used algorithm (uniproc.)
MM: Working Set Management
• Modified Page Writer thread
– Created at system initialization
– Writing modified pages to backing file
– Optimization: min. I/Os, contigous pages on disk
– Generally MPW is invoked before trimming
• Balance Set Manager thread
– Created at system initialization
– Wakes up every second
– Executes MmWorkingSetManager
– Trimming process WS when required: from current down to minimal WS for
processes with lowest page fault rate
– Aware of the system cache working set
– Process can be out-swapped if all threads have pageable kernel stack
MM: I/O Support
• I/O Support operations:
– Locking/Unlocking pages in memory
– Mapping/Unmapping Locked Pages into current address space
– Mapping/Unmapping I/O space
– Get physical address of a locked page
– Probe page for access
• Memory Descriptor List
– Starting VAD
– Size in Bytes
– Array of elements to be filled with physical page numbers
• Physically contiguous vs. Virtually contiguous
Memory Manager: Services
• Caller can manipulate own/remote memory
– Parent process can allocate/deallocate, read/write memory of child
process
– Subsystems manage memory of their client processes this way
• Most services are exposed through Windows API
• Services for device drivers/kernel code (Mm...)
Protecting Memory Attribute Description
PAGE_NOACCESS Read/write/execute causes access violation
PAGE_READONLY Write/execute causes access violation; read permitted
PAGE_READWRITE Read/write accesses permitted
PAGE_EXECUTE Any read/write causes access violation; execution of code is
permitted (relies on special processor support)
PAGE_EXECUTE_
READ
Read/execute access permitted (relies on special processor
support)
PAGE_EXECUTE_
READWRITE
All accesses permitted (relies on special processor support)
PAGE_WRITECOPY Write access causes the system to give process a private copy
of this page; attempts to execute code cause access violation
PAGE_EXECUTE_
WRITECOPY
Write access causes creation of private copy of pg.
PAGE_GUARD Any read/write attempt raises EXCEPTION_GUARD_PAGE
and turns off guard page status
Reserving & Committing Memory
• Optional 2-phase approach to memory allocation:
1. Reserve address space (in multiples of page size)
2. Commit storage in that address space
– Can be combined in one call (VirtualAlloc, VirtualAllocEx)
• Reserved memory:
– Range of virtual addresses reserved for future use (contiguous buffer)
– Accessing reserved memory results in access violation
– Fast, inexpensive
• Committed memory:
– Has backing store (pagefile.sys, memory-mapped file)
– Either private or mapped into a view of a section
– Decommit via VirtualFree, VirtualFreeEx
A thread‘s user-mode stack is constructed using this 2-phase approach: initial reserved size is 1MB, only 2 pages are committed: stack & guard page
Features new to Windows XP/2003 and
newer OS in Memory Management
• 64-bit support
• Up to 1024 GB physical memory
supported (2048 on 2008 R2)
• Support for Data Execution Prevention
(DEP)
– Memory manager supports HW no-execute
protection
• Performance & Scalability enhancements
Shared Memory & Mapped Files
• Shared memory + copy-on-
write per default
• Executables are mapped as
read-only
• Memory manager uses
section objects to implement
shared memory
(file mapping objects in
Windows API)
compiler
image
Physical memory
Process 1 virtual memory
Process 2 virtual memory
Virtual Address Space Allocation
• Virtual address space is sparse
– Address spaces contain reserved, committed, and unused
regions
• Unit of protection and usage is one page
– On x86, default page size is 4 KB (x86 supports 4KB or 4MB)
• In PAE mode, large pages are 2 MB
– On x64, default page size is 4 KB (large pages are 4 MB)
– On Itanium, default page size is 8 KB
(Itanium supports 4k, 8k, 16k, 64k, 256k, 1mb, 4mb, 16mb,
64mb, or 256mb) – large is 16MB
Large Pages
• Large pages allow a single page directory entry to map a larger region
– x86, x64: 4 MB, IA64: 16 MB
– Advantage: improves performance
• Single TLB entry used to map larger area
• Disadvantage: disables kernel write protection
– With small pages, OS/driver code pages are mapped as read only; with large pages, entire area must be mapped read/write
• Drivers can then modify/corrupt system & driver code without immediately crashing system
– Driver Verifier turns large pages off
– Can also override by changing a registry key
Data Execution Prevention
• Windows XP SP2 and newer OS support Data Execution Prevention (DEP)
– Prevents code from executing in a memory page not specifically marked as executable
– Stops exploits that rely on getting code executed in data areas
• Relies on hardware ability to mark pages as non executable, AMD NX or Intel XD
• Processor support: – About all CPU from Intel, AMD and VIA shipped in last 4 years.
Data Execution Prevention
• Attempts to execute code in a page marked no execute result in:
– User mode: access violation exception
– Kernel mode: ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY
bugcheck (blue screen)
• Memory that needs to be executable must be marked as such using
page protection bits on VirtualAlloc and VirtualProtect APIs:
– PAGE_EXECUTE, PAGE_EXECUTE_READ, PAGE_EXECUTE_READWRITE,
PAGE_EXECUTE_WRITECOPY
Mapped Files
• A way to take part of a file and map it to a range of virtual addresses
(address space is 2 GB, but files can be much larger)
• Called “file mapping objects” in Windows API
• Bytes in the file then correspond one-for-one with bytes in the region of virtual address space
– Read from the “memory” fetches data from the file
– Pages are kept in physical memory as needed
– Changes to the memory are eventually written back to the file (can request explicit flush)
• Initial mapped files in a process include:
– The executable image (EXE)
– One or more Dynamically Linked Libraries (DLLs)
Shared Memory
• Like most modern OS‟s, Windows provides a
way for processes to share memory
– High speed IPC (used by LPC, which is
used by RPC)
– Threads share address space, but
applications may be divided into
multiple processes for stability reasons
• It does this automatically for shareable
pages
– E.g. code pages in an EXE or DLL
• Processes can also create shared memory
sections
– Called page file backed file mapping
objects
– Full Windows security
compiler
image
Physical memory
Process 1 virtual memory
Process 2 virtual memory
Copy-On-Write Pages
• Used for sharing between process address spaces
• Pages are originally set up as shared, read-only, faulted from the common
file
– Access violation on write attempt alerts pager
• pager makes a copy of the page and allocates it privately to the process doing the
write, backed to the paging file
– So, only need unique copies for the pages in the shared region that are actually
written (example of “lazy evaluation”)
– Original values of data are still shared
• e.g. writeable data initialized with C initializers
Physical memory
Page 3
Page 1
How Copy-On-Write Works
Before
Process Address Space
Orig. Data
Process Address Space
Orig. Data
Page 2
Process Address Space
Physical memory
How Copy-On-Write Works
After
Process Address Space
Orig. Data
Page 3
Page 1
Page 2
Mod’d. Data
Copy of page 2
Shared Memory = File Mapped by
Multiple Processes
• Note, the shared region may be mapped at different addresses in the different processes
00000000
7FFFFFFF
User
accessible
v.a.s.
User
accessible
v.a.s.
Process A Process B
Physical Memory
Virtual Address Space (V.A.S.)
Process space contains:
– The application you‟re running (.EXE and .DLLs)
– A user-mode stack for each thread (automatic storage)
– All static storage defined by the application
User
accessible
Kernel-mode
accessible
}
}
Unique per
process
System-
wide
00000000
7FFFFFFF
80000000
FFFFFFFF
Virtual Address Space (V.A.S.)
Kernel-mode
accessible
User
accessible }
}
Unique per
process
System-
wide
• System space contains: – Executive, kernel, and HAL
– Statically-allocated system-wide data cells
– Page tables (remapped for each process)
– Executive heaps (pools)
– Kernel-mode device drivers (in nonpaged pool)
– File system cache
– A kernel-mode stack for every thread in every process
00000000
7FFFFFFF
80000000
FFFFFFFF
3GB Process Space Option • Only available on operating system newer
than Windows 2000 Server. – Can be activated from Boot.ini (Win 2k3, XP) or
BCD (Vista, 7, 2008)
• Provides 3 GB per-process address space
– Commonly used by database servers (for
file mapping)
– .EXE must have “large address space
aware” flag in image header, or they‟re
limited to 2 GB (specify at link time or with
imagecfg.exe from ResKit)
– Chief “loser” in system space is file system
cache
– Better solution: address windowing
extensions
– Even better: 64-bit Windows
Unique per process (= per appl.), user mode
.EXE code Globals
Per-thread user mode stacks
.DLL code Process heaps
Exec, kernel, HAL, drivers, etc.
00000000
BFFFFFFF
FFFFFFFF
C0000000
Unique per
process,
accessible in
user or kernel
mode
System wide,
accessible
only in kernel
mode
Per process,
accessible only
in kernel
mode
Process page tables, hyperspace
Physical Memory
• Maximum on Windows NT 4.0 was 4 GB for x86 (8 GB for Alpha AXP)
– This is fixed by page table entry (PTE) format
• What about x86 systems with > 4 GB?
– If CPU has PAE support can manage more than 64 GB (36 bits addressing)
• Windows 2000 added proper support for PAE
– Requires booting /PAE to select the PAE kernel
• Actual physical memory usable varies by Windows SKU.
Physical Memory Limits x86 x64 32-bit x64 64-bit
XP Home 4 4 n/a
XP Professional 4 4 16 GB
Vista Home Premium 4 4 16 GB
Vista Bus / Ent / Ultimate 4 4 128 GB
Seven Home Premium 4 4 16 GB
Seven Pro / Ent / Ultimate 4 4 196 GB
2008 R2 Standard n/a n/a 32 GB
2008 R2 Ent / Datacenter n/a n/a 2 TB
http://msdn.microsoft.com/en-us/library/aa366778(VS.85).aspx
74
Working Set
• Working set: All the physical pages “owned” by a process
– Essentially, all the pages the process can reference without incurring a page fault
• Working set limit: The maximum pages the process can own
– When limit is reached, a page must be released for every page that‟s brought in (“working set replacement”)
– Default upper limit on size for each process
– System-wide maximum calculated & stored in MmMaximumWorkingSetSize
• approximately RAM minus 512 pages (2 MB on x86) minus min size of system working set (1.5 MB on x86)
• Interesting to view (gives you an idea how much memory you‟ve “lost” to the OS)
– True upper limit: 2 GB minus 64 MB for 32-bit Windows
75
Working Set List
• A process always starts with an empty working set – It then incurs page faults when referencing a page that isn‟t in its working set – Many page faults may be resolved from memory (to be described later)
PerfMon Process “WorkingSet”
newer pages older pages
76
Birth of a Working Set
• Pages are brought into memory as a result of page faults
– Prior to XP, no pre-fetching at image startup
– But readahead is performed after a fault
• See MmCodeClusterSize, MmDataClusterSize, MmReadClusterSize
• If the page is not in memory, the appropriate block in the associated file is read in
– Physical page is allocated
– Block is read into the physical page
– Page table entry is filled in
– Exception is dismissed
– Processor re-executes the instruction that caused the page fault (and this time, it succeeds)
• The page has now been “faulted into” the process “working set”
77
Prefetch Mechanism
• First 10 seconds of file activity is traced and used to prefetch data the next time
– Also done at boot time (described in Startup/Shutdown section)
• Prefetch “trace file” stored in \Windows\Prefetch
– Name of .EXE-<hash of full path>.pf
• When application run again, system automatically
– Reads in directories referenced
– Reads in code and file data
• Reads are asynchronous, but waits for all prefetch to complete
• In addition, every 3 days, system automatically defrags files involved in each application startup
78
Working Set Replacement
• When working set max reached (or working set trim occurs), must give up pages to make room for new pages
• Local page replacement policy (most Unix systems implement global replacement)
– Means that a single process cannot take over all of physical memory unless other processes aren‟t using it
• Page replacement algorithm is least recently accessed (pages are aged)
– On UP systems only in Windows 2000 – done on all systems in Windows XP/Server 2003
• New VirtualAlloc flag in XP/Server 2003: MEM_WRITE_WATCH
PerfMon Process “WorkingSet”
to standby or modified
page list
79
Free and Zero Page Lists
• Free Page List – Used for page reads
– Private modified pages go here on process exit
– Pages contain junk in them (e.g. not zeroed)
– On most busy systems, this is empty
• Zero Page List – Used to satisfy demand zero page faults
• References to private pages that have not been created yet
– When free page list has 8 or more pages, a priority zero thread is awoken to zero them
– On most busy systems, this is empty too
80
Paging Dynamics
Standby Page List
Zero Page List
Free Page List
Process Working
Sets
page read from disk or kernel allocations
demand zero page faults
working set replacement
Modified Page List
modified page
writer
zero page
thread
“soft” page faults
Bad Page List
Private pages at process exit
81
Why “Memory Optimizers” are
Fraudware
Notepad Word Explorer System Available
Avail. RAM Optimizer
System Explorer Word Notepad
Available
Before:
During:
After:
Copyright Notice © 2000-2005 David A. Solomon and Mark Russinovich
• These materials are part of the Windows Operating
System Internals Curriculum Development Kit, developed
by David A. Solomon and Mark E. Russinovich with
Andreas Polze
• Microsoft has licensed these materials from David
Solomon Expert Seminars, Inc. for distribution to
academic organizations solely for use in academic
environments (and not for commercial use)
Microsoft, Windows Server 2003 R2, Windows Server 2008, Windows 7 and Window Vista are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries. The names of actual companies and products mentioned herein may be the trademarks of their respective owners. The information herein is for informational purposes only and represents the current view of Microsoft Corporation as of the date of this presentation. Because Microsoft must respond to changing market conditions, it should not be interpreted to be a commitment on the part of Microsoft, and Microsoft cannot guarantee the accuracy of any information provided after the date of this presentation. MICROSOFT MAKES NO WARRANTIES, EXPRESS, IMPLIED OR STATUTORY, AS TO THE INFORMATION IN THIS PRESENTATION.